1. Field of the Invention
This invention relates to a laser diode pumped solid state laser in which a solid laser crystal added with Pr.sup.3+ is pumped by a laser diode (semiconductor laser), a fiber laser in which a fiber having a core added with Pr.sup.3+ is pumped by a laser diode, and a fiber amplifier in which a fiber having a core added with Pr.sup.3+ is pumped by a laser diode to generate fluorescence and light entering the fiber is amplified by the fluorescence.
2. Description of the Related Art
As disclosed, for instance, in "Journal of Applied Physics" Vol. 48, No. 2, pp, 650 to 653 (1977), and "Applied Physics B58", pp. 149 to 151 (1994), there has been known a gas laser pumped solid state laser in which a solid laser crystal added with Pr.sup.3+ is pumped by a gas laser such as an Ar laser.
In such a gas laser pumped solid state laser, a laser beam in a blue spectral range having a wavelength of 470 to 490 nm can be generated by .sup.3 P.sub.0 .fwdarw..sup.3 H.sub.4 transition and a laser beam in a green spectral range having a wavelength of 520 to 550 nm can be generated by .sup.3 P.sub.1 .fwdarw..sup.3 H.sub.5 transition. Accordingly such a gas laser pumped solid state laser can be employed as a light source for recording a color image on a color photosensitive material.
As solid state lasers generating laser beams in the blue spectral range or the green spectral range, there have been known laser diode pumped solid state lasers in which a solid laser beam is converted to its second harmonic by a nonlinear optical crystal disposed in a resonator as disclosed, for instance, in Japanese Unexamined Patent Publication No. 4(1992)-318988.
Further there have been recently developed InGaN laser diodes and ZnMgSSe laser diodes which generate laser beams in the blue spectral range or the green spectral range.
When a laser generating a laser beam in the blue or green spectral range is employed as a light source for recording a color image, it is desired that the laser is small in both size and weight and is inexpensive. From this viewpoint, the aforesaid gas laser pumped solid state laser in which a solid laser crystal added with Pr.sup.3+ is pumped by a gas laser is disadvantageous since the gas laser is large in both size and weight and expensive.
Further the aforesaid laser diode pumped solid state laser in which a solid laser beam is converted to a laser beam having a shorter wavelength by a nonlinear optical crystal has a problem that it is difficult to operate at a high output power since the wavelength conversion efficiency is not sufficient at present. Further in the laser diode pumped solid state laser, an etalon or the like is disposed in the resonator in order to make the oscillation mode a single longitudinal mode, the resonator loss is increased, which also makes the solid state laser difficult to operate at a high output power.
Further the solid state laser requires a high accuracy temperature control in order to obtain phase matching in wavelength conversion, which deteriorates stability of output power. Further the nonlinear optical crystal and the etalon add to the number of parts of the solid state laser, which increases the manufacturing cost of the solid state laser.
In the InGaN laser diodes, since the oscillating wavelength becomes longer as the In content increases, it is theoretically possible to generate a laser beam in a blue spectral range having a wavelength of 470 to 490 nm or a laser beam in a green spectral range having a wavelength of 520 to 550 nm. However since crystallizability deteriorates as the In content increases, increase in the In content is practically limited and the wavelength of the laser beam generated by the InGaN laser diode can be increased up to about 450 nm at most.
Laser diodes having an active layer of InGaNAs or GaNAs can also generate a laser beam in the blue spectral range. In these laser diodes, the oscillating wavelength can be increased by doping As. However since crystallizability deteriorates as the As content increases, the wavelength of the laser beam generated by the laser diode at a practical output power can be increased up to about 450 nm to 460 nm at most.
Further the aforesaid ZnMgSSe laser diodes are disadvantageous in that they cannot continuously oscillate at room temperature at a wavelength shorter than 500 nm and their service life is about 100 hours at most.
Further there has been known a fiber laser in which a fiber having a fluoride core added with Pr.sup.3+ is pumped by a laser diode as disclosed in "Electronic Communication Society Technical Information": LQE95-30(1995) p.30 and "Optics communications" 86(1991) p.337.
Further there has been known a fiber amplifier in which a fiber having a core added with Pr.sup.3+ is pumped by a laser diode to generate fluorescence and light in the wavelength range of the fluorescence is amplified by energy of the fluorescence as disclosed also in the above papers.
Especially in the latter paper, there is described a Pr.sup.3+ -doped fiber laser pumped by an Ar laser which oscillates at 491 nm, 520 nm, 605 nm and 635 nm when pumped by a 476.5 nm laser beam.
Since these fiber laser and fiber amplifier can generate or amplify a laser beam in the blue or green spectral range, it is possible to form a light source for recording a color image on a color photosensitive material by use of the fiber laser and/or the fiber amplifier.
However the aforesaid fiber laser and the fiber amplifier pumped by an Ar laser requires a water cooling means when pumped by power of several watts to several tens of watts in order to record a color image, which increases the overall size of the system, shortens the service life and deteriorates the efficiency.